Magnesium and magnesium alloys are specifically useful for the manufacturing of many light weight components and of many critical components for severe applications, for example for the manufacturing of secondary structural elements for aircrafts as well as of components for vehicles and electronic devices, because of their light weight and strength.
One of the significant disadvantages of magnesium and magnesium alloys is their sensitivity for corrosion. Exposure to hazardous chemical conditions causes magnesium rich surfaces to corrode quickly. Corrosion is unaesthetic and reduces strength.
A method that is often used to improve the corrosion resistance of metallic surfaces is painting. When the metallic surface is protected by a thick paint layer from the contact with corrosive agents, corrosion is prevented. However, many types of paint do not bind well to magnesium and magnesium alloy surfaces.
Methods based on chemical conversion of an outer metallic surface using chromate solutions are well known in the art as being useful for treating magnesium and magnesium alloy surfaces to increase corrosion resistance and paint adhesion, see for example U.S. Pat. Nos. 2,035,380 or 3,457,124. Chromate containing coatings are mostly colored and excellent visible. However, the corrosion resistance of treated magnesium rich surfaces is typically very low—quite different from other metallic substrates coated with a chromate coating—and the environmental unfriendliness as well as the dangers for living beings of chromate solutions are definite disadvantages of these methods.
Several methods of metal surface treatment using non-chromate conversion coatings have been disclosed, for example in U.S. Pat. Nos. 5,292,549, 5,750,197, 5,759,629 and 6,106,901. Silane solutions are environmentally friendly and lend excellent corrosion resistance to treated metal surfaces. Silane from the solution binds to a treated metallic surface forming a layer to which commonly used polymers such as paint or adhesive may be further applied, see U.S. Pat. No. 5,750,197.
U.S. Pat. No. 6,777,094 teaches to provide a silane pretreatment on magnesium and magnesium alloys. Although the disclosed treatment offers excellent paint adhesion and corrosion protection, the coating is transparent and requires special on-line control methods.
Many of the present non-chromate treatment technologies are based on Group IV metals of the Periodical Table of Chemical Elements such as titanium, zirconium or hafnium, a source of fluoride ion and a mineral acid for a pH adjustment. For example, U.S. Pat. No. 3,964,936 discloses the use of zirconium, fluoride, nitric acid and boron to produce a uniform, colorless and clear conversion coating for aluminum. U.S. Pat. No. 4,148,670 teaches a clear conversion coating comprising zirconium, fluoride and phosphate. U.S. Pat. No. 4,273,592 concerns a coating comprising zirconium, fluoride and a C1-7 polyhydroxy compound, wherein the composition is essentially free of phosphate and boron. U.S. Pat. No. 6,083,309 refers to a coating comprising Group IV metals such as zirconium in combination with one or more non-fluoro anions while fluorides are specifically excluded from the processes and compositions above certain levels. The main lack of these conversion coatings is again the lack of a color and visibility, as the coatings are all clear and colorless or mostly colorless.
Recently disclosed well visible non-chromate conversion coatings include additionally to the metals of Group IV of the Periodic Table of Chemical Elements and to fluorides also any a special color providing component, such as an alizarine dye in U.S. Pat. No. 6,464,800 and such as permanganic acid and its water soluble salts in U.S. Pat. No. 6,485,580.
Permanganic acid is not preferred as its coloring effect is too strong and as its impurities are difficult to avoid and to remove. But the main lack of compositions containing permanganic acid or any of its salts is a low stability in contact with a magnesium rich surface so that it requires an addition of at least one sequestering agent and an extended use of chemicals.
The addition of organic dyes to process solutions usually leads to higher coating costs, to complicate compositions and to difficulties to control the process solution by optical methods like photometry.
Additionally, one critical disadvantage of non-chromate conversion coatings based on Group IV metals of the Periodic Table of Chemical Elements is the very low adhesion of the formed conversion coating to fluoropolymer coatings. Anodizing coatings or phosphate coatings are usually used as pretreatment coatings for magnesium rich surfaces, often prior to a PTFE coating.
Anodizing coatings or phosphate coatings are also used like pretreatment coatings prior to applying self-lubricant coatings like MoS2 or graphite containing coatings on metal sliding components and in forming technologies like deep-drawing or forging.
Anodizing coatings as well as most of the phosphate coatings are well visible on magnesium rich surfaces. However, as it is well known for one skilled in the art, thick crystalline phosphate conversion coatings often fail to form layers on magnesium surfaces showing sufficient corrosion resistance and paint adhesion. Providing an anodizing technology for magnesium rich surfaces requires a complicate and expensive equipment.
It would be highly advantageous to have a method for treating magnesium and magnesium alloys with a non-complicate and stable composition which allows to form a well visible coating layer which has at least the same corrosion resistance and at least the same adhesion of the conversion coating to paint coatings, powder coatings, e-coats, fluoropolymer containing coatings, self-lubricant layers like coatings containing MoS2 or graphite like conversion coatings and adhesives layers typically used in the art for magnesium rich surfaces.
It has now been found that aqueous compositions containing a fluorosilicon acid and optionally a pH adjustment agent form either invisible clear and mostly even colorless coatings or no coatings on surfaces on aluminum, aluminum alloys, steel and zinc, but the same compositions or modified compositions form well visible grey or black crack-free coatings with a mat non-metallic appearance on surfaces of magnesium or magnesium alloys.